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You need to figure out where it goes bad. You didn't say what model you have, and it makes a difference - so you either need to always tell us what you have or, better yet, put the info in your signature so we do not have to look.
But, the backup wiring is shown on Page 47 here: Lights: - ???Gary's Garagemahal. And, for the earlier years, as shown, each bulb has its own ground. So just because you replaced the bulb and socket doesn't mean you have a good ground. It usually grounds under a screw inside the tail light cavity, and may need to be cleaned.
Rezvani's Latest Post-Apocalyptic Monster Is a Ford F-150 Raptor Underneath
Slideshow: Called the Fortress, the 850-horsepower pickup combines Raptor underpinnings with military-inspired features, survival equipment, and a starting price of $285,000.